The present invention relates to a sheet heating device in which a sheet having an image thereon is delivered while being heated by a heater disposed in a sheet feed path, and more particularly to a sheet heating device suitable for an image forming apparatus such as a copying machine.
In a conventional sheet heating device used in an image forming apparatus such as a copying machine, in which an image of thermally fusible substances formed on a piece of sheet such as paper is heat-fixed, the sheet is clamped between sheet feed rollers to supply the sheet to a sheet guide path and fed through sheet discharge rollers for discharging the sheet to a discharge cassette. During a heat-fixing process of the conventional sheet heating device, air heated by a heater which is provided in the sheet heating device is circulated and impinged onto the sheet, or radiation heat of the heater is applied to the sheet to heat-fix the image on the sheet while the sheet is fed through the sheet guide path in the sheet heating device. After the heat-fixing process, the sheet is clamped and discharged by the discharge rollers.
FIG. 1 shows a conventional sheet heating device used in an image forming apparatus. This sheet heating device adopts an air heating method in which a heated air is applied to the sheet introduced into the sheet heating device to heat-fix the image on the sheet. In this device, two pairs of rotatable rollers (sheet feeding rollers 32, 33 and sheet discharge rollers 36, 42 are provided at both ends of a sheet guide path (plate) 50, and a cover case 56 is provided such that it covers the sheet guide path 50. The cover case 56 includes a heater 57 and a blower fan 60 for supplying air to the heater 57. The air supplied from the fan 60 is heated by the heater 57 and circulated along the inner wall of the cover case 56 and the sheet guide path 50 by convection (that is, the fan 60 is used to form a convection of heated air) to heat-fix the image on the sheet during a sheet feeding operation in the heat-fixing device.
In the conventional sheet heating device as described above, the sheet guide path 50 comprises an iron guide plate and therefore the temperature of the guide plate 50 is easily increased during the heat-fixing process. If the sheet 17 having the image thereon is introduced into the sheet heating device with the image surface erroneously placed face down, the thermally fusible substances forming the image are fused by contact with the iron guide plate of high temperature and is liable to adhere to the iron guide plate 50. This fusion of the substances causes the iron guide plate 50 to be stained and its friction resistance to be increased. As a result, the feeding of the sheet along the sheet guide plate 50 is prevented nd a sheet jam is liable to occur.
Further, in the conventional sheet heating device, the heated air is circulated in the cover case 56 such that it flows along the inner wall of the cover case 56 and the sheet guide plates 50 as shown in FIG. 1. Accordingly, the temperature of the heated air is higher beside the sheet discharge rollers 36, 42 than beside the sheet feeding rollers 32, 33, so that the sheet is not effectively heated beside the sheet feeding rollers 32, 33 and the heat-fixing of the sheet is inhomogeneously made.
Still further, in the conventional sheet heating device as described above, during the heat-fixing process of the sheet, the heated air would be leaked from the cover case 56 to heat the sheet feed/discharge rollers (32, 33) and (36, 42). As a result, the sheet would be rapidly heated at the sheet feed and discharge rollers. In this case, the thermally fusible substances forming the image are over-heated, so that they are boiled and bubbles are formed therein to thereby degrade a quality of the image, or the substances are over-heated, so that they are fused at the feed and discharge rollers and would adhere to the 4 rollers to thereby form image damages such as flaw and breakdown and deteriorate the quality of the image.